1. Field of the Invention
This invention relates generally to improved ester compositions, and more particularly to fatty acid esters of hydroxyalkyl sulfonate salts, referred to in the industry as sodium cocoyl isethionate (SCI). The esters are useful for personal care cleansing products, such as bar and liquid soaps, skin and hair care products.
2. Description of the Related Art
Esters and acids are known for a variety of different applications for cosmetic, pharmaceutical and medicinal purposes.
For example, U.S. Pat. No. 2,806,044 to WEIL et al. discloses detergent compositions prepared by the direct esterification of an .alpha.-sulfonated fatty acid with hydroxyalkanesulfonate. Since the .alpha.-sulfonated acid reagent is strongly acidic, no catalyst is required for production of the esters. Esterification is conducted under conditions such that water produced is removed from the reaction as fast as formed. Removal of the water may be done by use of an inert solvent that distills azeotropically with water.
U.S. Pat. No. 2,923,724 to ANDERSON et al. discloses a process for preparing ester type anionic surface active agents wherein a critical feature of the process is the presence of a phosphorous containing compound as a catalyst (Col. 2, lines 12-15). Anderson et al. compares the reactive components without a catalyst (Example 1) with the preferred process in which a catalyst is employed (Example 2). In Example 1, coconut oil fatty acid heated to 200.degree. C. was mixed with sodium isethionate. The mixture was heated to 210.degree. C. under an inert atmosphere of carbon dioxide. The reaction contained 20% of active material after two hours; the reaction rate was approximately 34% at the end of 3.5 hours. Catalysts are used in the process to improve reaction rates.
U.S. Pat. No. 5,523,432 to RATHS et al. discloses a process for the production of quaternary ammonium salts of fatty acid hydroxyalkanesulfonic acids, wherein the process is carried out at considerably lower temperatures than known process (at temperatures of from about 60.degree. C. to about 120.degree. C.) and without a catalyst. Water of solution and water of reaction are removed from the reaction mixture by distillation. Raths et al. points out that the advantage of carrying out the reaction without a catalyst is that there is thus no need to remove the catalyst from the reaction product and no catalyst residues are present in the reaction product. Catalysts also contribute to discoloration of the products. Raths et al. does not teach or suggest producing fatty acid esters using sodium hydroxysulfonate, which has a high melting point. Rather, Raths et al. discloses production of ammonium or amine salts using hydroxysulfonic acid, which has a lower melting point, and using solvents for conversion into amine salts.
U.S. Pat. No. 4,515,721 to LOGIN et al. discloses a process for preparing fatty acid esters of hydroxyalkyl sulfonic acid salts (i.e., hydroxyalkyl sulfonates) by heating an excess of the fatty acids with the hydroxyalkyl sulfonate, i.e., sodium isethionate, to a temperature of 200.degree. C. to 250.degree. C. until the water of condensation is removed. The hot crude ester is then quenched by immersion in an excess of cooled liquid such as an alcohol solvent, in which the ester product is substantially insoluble but in which unreacted, excess fatty acids are soluble. The resulting slurry is filtered to separate the relatively pure ester from the quenching liquid containing dissolved free fatty acid. The method of this patent recites a temperature range of 200.degree. C. to 250.degree. C., but all of the examples appear to be run at 250.degree. C.
U.S. Pat. No. 5,300,665 to TRACY et al. discloses preparation of fatty acid esters of hydroxyalkylsulfonates (i.e., sodium cocyl isethionate) by heating an excess of the fatty acids with the sulfonate in the presence of a reaction promoter (catalyst) at a temperature between about 200.degree. C. to 250.degree. C. in a substantially oxygen-free atmosphere until the water of condensation is removed. The excess fatty acid is removed by distillation and the isolated fatty acid ester is cooled to minimize decomposition and color degradation.
U.S. Pat. No. 3,420,857 to HOLLAND et al. and U.S. Pat. No. 3,420,858 to McCRIMLISK disclose methods for preparing fatty esters of hydroxysulfonates to obtain products which have reduced amounts of esters of higher molecular weight fatty acids and unreacted lower molecular weight fatty acids. The methods comprise continuously supplying to the reaction vessel, fatty acid reactants of a composition corresponding to fatty acids volatilized during the course of the reaction (in order to reduce the proportion of esters of relatively higher molecular weight fatty acids) and utilizing an improved stripping process to reduce the lower molecular weight fatty acid content. The method includes heating a mixture of an hydroxyalkylsulfonate and fatty acids to a temperature between about 390.degree. F. to 500.degree. F. (about 199.degree. C. to 260.degree. C.) The examples are run at temperatures of at least 450.degree. F. (about 232.degree. C.). The patents note that temperatures below 232.degree. C. significantly reduce reaction rates. While it is preferred to employ a reaction promoter to boost yields of the desired product, promoters are optional. In the absence of promoters, higher temperatures, such as up to about 500.degree. F., (about 260.degree. C.), are usually necessary to avoid premature cessation of the reaction. (Holland, Col. 2, lines 40-42; McCrimlisk, col. 2., lines 15-23). Neither McCrimlisk nor Holland teach or suggest a process in which there is none to negligible change in the molecular weight distribution of the SCI product.
U.S. Pat. No. 3,429,16 to HOLT et al. discloses a method for making esters of hydroxysulfonates in which the hot hydroxy-sulfonate esters are cooled from temperatures of about 350.degree. F. to 500.degree. F. (about 177.degree. C. to 260.degree. C.) to a temperature below about 330.degree. F. (about 165.6.degree. C.). At this point the reaction is quenched by injecting cold water. Reaction promoters are optionally provided to achieve acceptable conversion levels. In the presence of such reaction promoters, the reaction may be carried out at temperatures of 390.degree. F. to 465.degree. F. In the absence of such promoters, higher temperatures, such as up to about 500.degree. F. (about 260.degree. C.) are usually necessary to avoid premature cessation of the reaction.
U.S. Pat. No. 3,394,155 to CAHN et al. discloses a method for preparing esters of predominantly coco fatty acid and a hydroxy sulfonate by direct esterification which minimizes the residues of unreacted fatty acids of C.sub.8 -C.sub.12 carbon chain lengths. This is achieved by adding the fatty acid reactant in two steps. First, coco fatty acids containing substantial quantities of the C.sub.8 -C.sub.12 fatty acids are added in sufficient quantities to provide the desired proportion of coco esters in the final product, but insufficient to completely consume the hydroxy sulfonate. In the second step sufficient additional fatty acid is added to provide a high conversion of the hydroxy sulfonate, the additional fatty acid, however, containing substantially lower proportions of the C.sub.8 -C.sub.12 fatty acids. In the absence of a reaction promoter, a temperature of about 200.degree. C. to about 250.degree. C. is required. If a reaction promoter is employed, the maximum reaction temperature need not exceed about 240.degree. C.
U.S. Pat. No. 3,320,292 to CAHN et al. discloses preparation of sulfonated fatty acid ester surface-active agents. Examples 3 and 4 show that when coconut oil fatty acids are mixed with sodium isethionate in the presence of a catalyst of zinc oxide or zinc soap, esterification proceeds at lower temperatures (about 230.degree. C.) and shorter times (1 to 1.5 hours) with higher yields (90%-97%). In Example 1, when no catalyst is used, esterification proceeds at a temperature of about 240.degree. C.-251.degree. C. and the reaction is substantially complete in approximately 2 hours, with conversion percentages of between 87%-89%.
U.S. Pat. No. 3,092,264 to VAN ALPHEN et al. discloses preparation of a mixture of fatty acyl-oxyalkane sulphonates. In discussing the prior art in col. 1, lines 14-30, the '264 patent notes that merely heating together a fatty acid and a hydroxyalkane sulphonate such as an isethionate (without a promoter) has several disadvantages. Excessive foaming occurs necessitating the use of very large reaction vessels, vigorous stirring is required to ensure adequate mixing, reduced pressure must be maintained throughout the reaction, and the products are typically discolored.
U.S. Pat. No. 4,536,338 to URBAN et al. discloses a method for preparing fatty acid isethionate soaps through direct esterification wherein the catalyst is quenched by an alkaline compound at the end of the esterification and the traditional stripping step to remove excess C.sub.5 -C.sub.12 fatty acids following esterification may be eliminated. Urban et al. further teach that the choice of catalyst effects chain length distribution of the isethionate ester product.
U.S. Pat. No. 2,821,531 to BRITTON et al. discloses a process for making 2-sulfoethyl esters of fatty acids wherein a mixture of a salt of isethionic acid and a fatty acid chloride, i.e. an acylchloride of a fatty acid such as fatty acids derived from coconut oil, are admixed at temperatures between 135.degree. C. and 170.degree. C. during which hydrogen chloride is usually withdrawn from the reaction at about the rate it is formed. The process is employed to react salts of isethionic acid with fatty acid chlorides to yield the corresponding isethionate ester.
U.S. Pat. No. 3,150,156 to LAMBERTI discloses a catalytic process for preparing N-acyl taurates, wherein when a catalyst is not used, unsatisfactory results are obtained (see TABLE I).
U.S. Pat. No. 4,405,526 to LAMBERTI et al. discloses a process for producing esterified fatty acid isethionate by reacting a fatty acid with an alkali metal isethionate in the presence of a catalyst comprising a mixture of ZnO and an organic sulfonic acid and heating at about 200.degree. C. to about 255.degree. C.
U.S. Pat. No. 5,384,421 to DAY et al. discloses a process for making sodium acylisethionates, wherein there is direct esterification of a fatty acid with one or more salts of a selected hydroxyalkanesulfonic acid in the presence of a catalyst. The process makes fatty acid esters of hydroxyalkyl sulfonates and may be used at temperatures below 200.degree. C. in the presence of paraffin wax to lower the viscosity so that complete condensation can be achieved. There is hydrolysis of the ester. Stearic acid and NaOH are added to the cooled reaction mass. Water is mixed with the cooled reaction mass to form a pumpable fluid.
U.S. Pat. No. 5,434,276 to WALELE et al. discloses a process for making N-acyl taurides. An alkali metal borohydride acts as a catalyst in the reaction, although such may act as a reducing agent to promote the reaction.
However, among the foregoing patents, none disclose or suggest a process for the production of fatty acid esters of hydroxyalkyl sulfonate salts, wherein the fatty acids are maintained in a closed system and returned to the reactor vessel, and water is continuously removed without requiring a catalyst for the process or alkalies to quench the catalyst. The reaction is maintained fluid throughout. Neither are any of the foregoing patents directed to compositions of fatty acid esters of hydroxyalkyl sulfonate salts having superior properties, superior color, odor and activity integrity, with none or negligible change in the molecular weight distribution and with high conversion of sodium isethionate.